1. Field of the Invention
The present invention relates to wood composites comprising wood or wood-based products and reinforcing laminates. More specifically, the invention relates to a wood composite having an adhesive for forming a strong bond between the wood or wood-based products and reinforcing laminates. More specifically, the invention relates to a fiber reinforced polymer (FRP) composite with a thermoset polyurethane resin matrix bonded to a wood or wood-based product by means of an emulsion polymer isocyanate (EPI) adhesive.
2. Description of the Prior Art
Reinforced glulam/gluelam beams and other reinforced wood products have been used with limited success. One of the handicaps in their performance is the successful transfer of the load to the reinforcement. U.S. Pat. No. 5,362,545 (Tingley) has taught that an amount of synthetic fiber that does not adhere well to the matrix of the reinforcement can be added to the surface so that when abrasion occurs those fibers will protrude beyond the surface of the reinforcement (referred to as “hairing up”) and serve as a means to bond the reinforcement to the wood with a non-epoxy glue, such as resorcinol. Therefore, the surface of the reinforcing panel must be treated so that those fibers closest to the surface of the panel are caused to “hair up.” Thus, special preparation of the reinforcement is required to properly bond the reinforcement to the wood using resorcinol, which increases the time and cost associated with assembling reinforced glulam beams together.
We have also been taught by U.S. Pat. No. 6,179,942 (Padmanabhan) that a reactive hot melt adhesive can be used under specific conditions to bond a reinforcement to wood. The prior art of surface preparation by sanding, etching, abrading or plasma treating the surfaces has also been used in many situations to treat the surface of an FRP material to help other materials adhere to it.
Regarding the adhesion of a reinforcement to wood, the International Code Council (ICC) has issued performance requirements in the nature of glue bonding specifications. The ICC is a nonprofit organization dedicated to developing a single set of comprehensive and coordinated national model construction codes used to construct residential and commercial buildings, including homes and schools. Most U.S. cities, counties and states that adopt codes choose the International Codes developed by the ICC. The ICC has issued AC280, which allows a computer program model to predict beam values for fiber-reinforced-polymer glued-laminated timber and replaces massive testing (Acceptance Criteria For Fiber-Reinforced-Polymer Glued-Laminated Timber Using Mechanics-Based Models, AC280, Mar. 1, 2005, ICC Evaluation Services, Inc.). AC280 also specifies other performance requirements including adhesive bonding specifications for bonding FRP to FRP or FRP to wood. Companies develop their own codes in relation to specific instructions and materials that meet the AC280 criteria and general guidelines. These company codes can be licensed to beam builders for construction projects using beams.
With regard to adhesive qualification, AC280 also identifies additional standards which have been set by such agencies as ASTM International (American Society for Testing and Materials) and ANSI (American National Standards Institute). Specifically, an adhesive used to bond FRP to FRP or FRP to wood must conform to ASTM D 2559 (Standard Specification for Adhesives for Structural Laminated Wood Products for Use under Exterior (Wet Use) Exposure Conditions) and ANSI A190.1 (for wood products—Structural Glued Laminated Timber).
The University of Maine has developed a code under the AC280 guidelines known as “Relam” which recites specific instructions for the glulam beam industry in relation to a method for fastening of wood and wood based products to reinforced laminates. However, the “Relam” program has not garnered widespread acceptance by the glulam beam industry because of questions that the program does not work as promoted.
American Laminators also owns a code under the AC280 guidelines for making glulam beams. The code is PFC-5100. However, the code is written to accept only reinforcements that have been approved by one specific lab organization that is no longer in business.
Adhesives have been developed and used successfully for the bonding of wood together to meet ICC AC280, ASTM D-2559 and ANSI A190.1 requirements. EPI adhesives have been developed and used for the bonding of wood to wood and have been recommended to use in bonding wood to painted FRP and painted metal. The use of EPI adhesive for use in wood to wood applications has also been documented in NER-165 ICC Evaluation Services Inc. Legacy Report of Feb. 1, 2002. The ICC NER-165 from Ashland Specialty Chemical Company provides for their EPI adhesives to be used for bonding painted fiberglass to waferboard, OSB, or plywood. They do not elaborate on the surface preparation of the fiberglass prior to painting but normal practice would involve sanding or some other surface preparation to allow the paint to stick.
Although the bonding of composite FRP to composite FRP has also be done successfully, the bonding of wood to composite FRP to meet the ICC-AC280, ASTM D-2559 and ANSI A190.1 requirements has been problematic.
The bonding of wood to composite FRP has been problematic for a number of reasons. A number of non-epoxy adhesives have been used but trials have shown that the bond created between the composite FRP and wood was not strong enough. In the alternative, epoxy adhesives have been proposed for wood to composite FRP bonding due to their additional strength. However, the use of epoxy adhesives in wood to composite FRP bonding includes a number of drawbacks. Epoxy is very costly, difficult to apply to the composite FRP and wood, and has sometimes failed due to moisture. The closed (nonporous) smooth surface of the FRP did not allow the adhesive to form a strong mechanical bond to the FRP because the adhesive cannot penetrate into any surface irregularities and pores of the FRP. The reason for this is that two different types of surfaces (FRP and wood) were not subject to the same positive adhesive characteristics. Although the mechanical bond of the adhesive to the wood was strong, the bond of the same adhesive to the FRP was inadequate and delamination would occur between the bond of the FRP and the adhesive, causing the FRP to also delaminate from the wood. Special manufacturing techniques to provide a porous and rough surface to the reinforcements to enhance bonding have been employed such as “hairing up” as described above. However, these methods are difficult and have not been expanded upon.
Some adhesives that are currently used in FRP to wood bonding that require surface preparation of the FRP include resorcinol and hot melt polyurethane. Resorcinol is also approved and is used in wood to wood applications, but resorcinol will not pass the AC280 tests when used with current reinforcements unless a special reinforcement surface preparation is performed. Hot melt polyurethane is used for some reinforcement to wood such as the truck floor but even then with most reinforcements it will require a surface treatment and the hot melt adhesives have not been approved for the ICC beam requirements.
In Forest Products Journal, Vol. 44, No. (5), pp. 62-66, 1994, Gardner et el. examined several different adhesive systems for use in glue-laminated wood structures. These included resorcinol formaldehyde (RF), an emulsion polymer isocyanate (EPI), and an epoxy resin. For composite joints fabricated from glass-reinforced vinylester or polyester FRPs and yellow poplar, the highest shear strengths were obtained using the RF adhesives, followed by the epoxy and EPI systems. The RF adhesive was also found to produce the greatest percentage of dry test failures within the wood matrix for wood-vinylester FRP matrix systems with values exceeding 90%. However, polyester-wood joints were found to produce almost 100% cohesive failures within the FRP matrix under dry conditions. Wet tests produced only 20 to 40% wood failures for vinylester-wood joints and 80% FRP matrix failures for polyester/wood joints. Therefore, the use of an EPI adhesive for use in glue-laminated wood structures has been avoided since it did not perform as well as compared to RF adhesives and even epoxy adhesives.
The type of FRP will also affect how an adhesive bonds to the FRP. It is desirable to have the same or similar positive adhesive characteristics between the FRP and the adhesive which would result in a chemical bond. Existing patents and procedures for reinforcements bonded to wood do not include the use of a thermoset polyurethane resin matrix in the reinforcement. Thermoset polyurethane resin has a number of desirable characteristics including good flexibility, elongation and resistance to corrosion. However, the use of thermoset polyurethane resin for reinforcements has been thought to be too flexible, too hard to use as a matrix in the reinforcement, and to display poor bonding to adhesives. It should be noted that there is a difference between thermoplastics and thermosets. Thermoplastics usually contain additives to change the properties of the material such as polypropylene while thermosets usually contain catalysts that change the state of the material at the molecular level. Thermoplastics can be re-melted and recycled fairly easily. Thermosets typically are cured and molded into shape and are not recycled as easily. U.S. Pat. No. 6,749,921 (Edwards et al.) specifically states that the use of a thermoplastic polyurethane composite provides an avenue for the shaping of and hammering nails into the wood composite, which are not possible using fiber-reinforced thermoset composites due to their brittleness. Therefore, Edwards et al. teaches away from the use of a thermoset polyurethane composite in the wood composite industry.
Accordingly, there is a need for an adhesive for use in glue-laminated wood structures that does not require surface preparation of the wood structure or the FRP to form a strong bond between the wood structure and the FRP. There is also a need for an FRP which will form a strong bond with the adhesive so the FRP may be attached to a wood structure.